The present invention relates to an improved powder coating composition comprising a solid particulate film-forming mixture of a polymer containing reactive functional groups and a curing agent having functional groups reactive with the functional groups of the polymer, which components are well known in the art, wherein the improvement comprises an organic polysiloxane having various reactive functional groups reactive with the functional groups of the polymer and/or the curing agent. More particularly, this invention relates to improved powder coating compositions which provide enhanced filiform corrosion resistance.
Powder coating compositions for use in painting are extremely desirable. Such coating compositions greatly reduce and can even eliminate the organic solvents used in liquid paints. When the powder coating composition is thermally cured, little, if any, volatile material is given off to the surrounding environment. This is a significant advantage over liquid paints in which organic solvent is volatilized into the surrounding atmosphere when the paint is cured by heating.
A particular problem which often results from the use of powder coatings, particularly over aluminum substrates, is filiform corrosion which is a type of localized corrosion that affects painted metals (usually steel, aluminum and magnesium). Filiform corrosion generally occurs in wet environments at the site of a surface defect in the presence of soluble ionic species. As described in Filiform Corrosion in Polymer-coated Metals, A. Bautista, PROGRESS IN ORGANIC COATINGS 28 at pages 49-58 (1996), this deterioration process gives rise to corrosion products which are characterized by a filamentous, worm-like appearance under the coatings. The "filaments" typically exhibit an arborescent structure and grow directionally under the coating.
As filiform corrosion results in delamination of an organic coating from a metal substrate, thereby exposing the metal to the environment, it has become a matter for increasing concern in the areas of automotive, industrial and architectural coatings. Accordingly, it is desirable to provide a powder coating composition with improved filiform corrosion. It has been found that incorporation of certain organic polysiloxanes having functional groups reactive with the functional groups of the polymer and/or the curing agent improves the filiform corrosion resistance of the powder coating composition.
Polysiloxanes with hydroxyl functional groups (i.e., polysiloxane polyols) are well known in the art. Japanese Patent Publication 48-19941 describes polysiloxane polyols which are obtained by the dehydrogenation reaction between a polysiloxane hydride and an aliphatic polyhydric alcohol or polyoxyalkylene alcohol to introduce the alcoholic hydroxy groups onto the polysiloxane backbone. In practice, however, it is difficult to obtain an industrially significant yield of such polysiloxane polyols because such a hydrosilylation reaction readily gels. Another problem encountered with this hydrosilylation reaction is the difficulty in obtaining a solvent capable of dissolving both reactants. Strongly hydrophilic alcohols such as polyglycerols are highly soluble in alcohols and water, but insoluble in hydrocarbon solvents. Polysiloxanes, however, are generally only soluble in hydrocarbon solvents such as toluene or n-hexane.
U.S. Pat. No. 4,431,789 to Okazaki et al. discloses a polysiloxane polyol which is obtained by the hydrosilylation reaction between a polysiloxane containing silicon hydride and a polyglycerol compound having an aliphatically unsaturated linkage in the molecule. Examples of such polyglycerol compounds are those obtained by the reaction of allyl alcohol and glycidol or by the reaction of diglycerin and allyl glycidyl ether. This reaction, a so-called hydrosilylation reaction, is the addition reaction between an organosilicon compound having a hydrogen atom directly bonded to the silicon atom, i.e., a polysiloxane hydride, and an organic compound having aliphatic unsaturation in the molecule carried out in the presence of a catalytic amount of a Group VIII noble metal. The hydrosilylation reaction can proceed readily in the presence of an alcoholic solvent which can dissolve both reactants. The resulting polysiloxane polyols are useful in coatings as non-ionic surface active agents.
U.S. Pat. No. 5,260,469 discloses butoxylated polysiloxane polyols which are disclosed as being useful in cosmetics. U.S. Pat. No. 5,248,789 discloses epoxy functional polysiloxanes which are formed by reacting a polysiloxane-containing silicon hydride with allyl glycidyl ether.
U.S. Pat. No. 5,916,902 discloses polysiloxane polyols obtained by the hydrosilylation of a polysiloxane containing silicon hydride with an alkenyl polyoxyalkylene alcohol. These polysiloxane polyols can contain two or more primary hydroxyl terminal functional groups per pendant group.
U.S. Pat. No. 5,939,491 discloses acetoacetate functional polysiloxanes and coating compositions containing such. The acetoacetate functional polysiloxanes are obtained by the transesterification of a polysiloxane polyol with an acetoacetate. The coating composition taught by this reference comprises an acetoacetate functional polysiloxane, a polyamine or blocked polyamine, and, optionally a polyacrylate curing agent. This coating composition is liquid in form and typically prepared as a two-pack, ambient-cured system.
U.S. Pat. No. 5,939,491 discloses curable compositions comprising an organic polysiloxane containing various reactive functional groups and a curing agent containing functional groups which are reactive with the functional groups of polysiloxane. The organic polysiloxanes of this reference contain reactive functional groups such as OH, COOH, NCO, carboxylate, primary and secondary amine, amide, carbamate and epoxy functional groups.
These polysiloxanes are obtained by further reacting the hydroxyl groups of the polysiloxane polyols with other groups to provide various reactive functional groups pendant from the polysiloxane backbone. Such reactive functional groups allow incorporation of the polysiloxane moiety into curable compositions which can contain a variety of reactive components, including a variety of curing agents.